I thought this might be a good way to help others when they are kitting or restoring mopeds. Post your moped type, jetting info, cylinder type, exhaust and air box info. Make notations of anything else that may be pertinent.

PLEASE not post pictures or ask questions here. Either start a new thread or use the PM function.

(Added by jd on March 7, 2008:)Here are some basic guidelines to follow if you're trying to figure out how to choose the proper jet for your moped:

First, take a look at page 17 in the Tomos A35 Repair Manual in the Moped Repair library. There are several pictures of spark plugs that can help you diagnose problems. It doesn't matter if you have a Tomos or not.

Second, learn how to do a "plug chop." With a plug chop, you can use the spark plug to evaluate your jet's performance and eliminate all doubt. Start with a new spark plug. Then warm up the engine, run it flat out (riding, not on a stand) for at least 30 seconds (a minute is better), and shut it off immediately with the kill switch. DO NOT allow it to return to idle first. Remove the spark plug (it will be HOT) and "read" it. Compare it to the pictures in the Tomos manual.

Generally, you want to look at the color of the insulator. In a perfect mix, it should be a tan, or maybe "coffee with milk" color. If it's white, you're too lean. If it's black or sooty, you're too rich.

A lean condition may result in your engine's best performance, but it will run extremely hot, cause detonation and ultimately result in cylinder and/or piston damage. The piston will expand faster than the cylinder and it will eventually get caught in there, referred to as a "seize." A "soft seize" will be temporary until the piston cools, but it could leave some serious scoring in the cylinder wall. A full seize will lock the piston up good, and may require major repairs.

A rich condition , on the other hand, will not damage your engine, although you may foul some spark plugs. The engine will appear to run OK until you open the throttle wide, in which case it will start to stumble, bog or "four-stroke." It's like trying to run a warm engine with the choke on.

Therefore, it's always better to start at the rich end with the largest reasonable jet, do a plug chop, and then decrease the jet size gradually, each followed by a plug chop, until you get a good reading on the plug.

Lastly, be conscious of the ambient outside temperature. Colder air is denser and will tend to cause any two-stroke engine to run leaner. Hot air does the opposite. If you do your plug chops on the hottest day of the year, you may end up underjetting and still being too lean when the weather cools. Doing this on a very cold day could results in an overjet.

The most important thing to remember is that you never want to use a jet that is too small. It will be a lot of fun for a very short time. It's always better to err on the size of too rich with an oversized jet.

2006 Streetmate Type R
A-55 engine with Airsal A-55 kit - stock head
Stock carb and intake
Opened air box inlet as wide as I could
Fabricated a new snorkel that was similar to the original - just wider (tip from Tomos03)
Technigas Next exhaust
Jetting - 58
25 idle jet
alt - 930 feet

I started the break in with a 70 which was quite rich. I am now back down to the 66 that came stock with the 13.13. and my plug is nice and lighter brown now. You may need a bigger jet if running a fatter expansion chamber exhaust like proma circuit or bigger kit like the malossi 70cc. One guy here in Seattle runs just fine with that setup and the 66, says his plug chops are coming out tan, but the guy on http://www.kinetictfr.com/blog/ ran lean and soft-seized.

The stock jet was a 45 which works fine in India or for hot weather or high altitude, but it's usually too lean for the US. Cosmopolitan motors usually up-jets to a 48 before they ship on their stock new bikes for the USA.